Liquid crystal used in the LCD (Liquid Crystal Display) is not a light emitting material which generates its own light by itself but a photospheric material by which an amount of light coming from the outside is modulated to display an image on a screen. Accordingly, the LCD device requires a separate light source, namely, a backlight unit, for radiating light onto an LCD panel. Any complex body capable of irradiating light from a rear surface of a liquid crystal module is called a backlight unit (BLU) or a backlight assembly.
The backlight unit (BLU) is generally divided, according to a position of a light source with respect to a display surface, into an edge type BLU that a lamp is installed on a lateral surface of a liquid crystal panel thus to provide light to a liquid crystal layer, and a direct type BLU that a lamp is installed below a liquid crystal panel thus to directly provide light to a liquid crystal layer.
According to the edge type BLU, a lamp is installed on a side surface of a liquid crystal panel thus to provide light to a liquid crystal layer through a reflector and a light guide panel. Accordingly, the edge type BLU has a thin thickness thereby to be mainly applied to a liquid crystal panel of small and medium areas such as a notebook computer, etc.
According to the direct type BLU, light emitted from a lamp is directly supplied to a liquid crystal layer. Accordingly, the direct type BLU can be applied to a liquid crystal panel of a large area, and a high brightness can be achieved. Therefore, the direct type backlight is mainly used to fabricate a liquid crystal panel for an LCD TV.
FIG. 1 is a schematic view illustrating a configuration of a conventional LCD backlight unit.
Referring to FIG. 1, the conventional LCD backlight unit includes a light source (10), a light guide plate (20), two diffusion sheets (30) (a first diffusion sheet for diffusing light coming from the light source and a second diffusion sheet for diffusing the light diffused by the first diffusion sheet to enhance the uniformity of the light), and a brightness enhancement film (BEF) (40) for enhancing brightness of the light diffused by the first and second diffusion sheets. In addition, the conventional LCD backlight unit may further include a reflection plate (50), a mold frame (not shown) and protective sheet (not shown).
The quality of screen image is dependent on the optical characteristic of light generated in the edge type BLU or the direct type BLU. Therefore, optical sheets are parts through which the light emitted from BLU become uniform and bright.
Particularly, since the LCD has advantages of slimness, lightweight, low power consumption and low driving voltage as compared with other display apparatuses, the LCD has been extensively employed in various industrial fields. To this end, however, the number of lamps has been reduced and the LED BLU has been employed. The optical sheet in BLU therefore acutely requires improvement in function of diffusion and brightness. A gain-diffuse plate having randomly distributed beads and being formed by using UV-resin, as shown in FIG. 2. is currently used as an optical sheet satisfying both the brightness and diffusion characteristic simultaneously.
The conventional random type gain diffuse plate is however still short of satisfying the requirements of low power consumption, high brightness and high diffusion characteristics in LCD despite the continuous developing efforts to improving brightness and diffusion function. The reason is that it difficult to control the optical characteristic according to the beads size due to the random distribution of beads. The disadvantage is caused by uncontrollability of packing factor and sag (lens height/lens diameter) of lens which are the most important factor for brightness and diffusion characteristics. As a result, the gain diffuse plate of regular shapes is currently required, but a gain diffuse plate having a large area is hardly commercialized yet. The reason is that the use of regular shaped or regular patterned plate makes moire phenomenon occur due to repeated periodicity on the LCD panel, which in turn causes decreased luminance of LCD panel and imbalance in a screen image, and which acts as limitation to application of the regular patterned sheets.
And, light bulbs such as fluorescent lamp, incandescent lamp was used as a light source at home. So quality of light is not good, and dazzling is brought about because the light bulbs are driven by commonly used alternating current frequency, which cause user's eye to be tired. To reduce the aforementioned disadvantage, optical sheet which can improve a luminance of light bulbs and reduce the eye-tiredness and prevent a amblyopia by being positioned at the light outlet, is required.